Replace a lot of 'static const' with 'constexpr' or 'const'.

gm/xfermodeimagefilter.cpp

Index: gm/xfermodeimagefilter.cpp
diff --git a/gm/xfermodeimagefilter.cpp b/gm/xfermodeimagefilter.cpp
index ee6822a3f44f9d3e86c21273c9882fd6a7b08141..0ff0ea9dd1341cf470327ed7418919df22ff7672 100644
--- a/gm/xfermodeimagefilter.cpp
+++ b/gm/xfermodeimagefilter.cpp
@@ -148,7 +148,7 @@ protected:
             y += fBitmap.height() + MARGIN;
         }
         // Test cropping
-        static const size_t nbSamples = 3;
+        constexpr size_t nbSamples = 3;
         SkXfermode::Mode sampledModes[nbSamples] = {SkXfermode::kOverlay_Mode,
                                                     SkXfermode::kSrcOver_Mode,
                                                     SkXfermode::kPlus_Mode};